1. Field of the Invention
The present invention relates to electronic hearing devices and electronic systems for sound reproduction. More particularly, the present invention relates to noise suppression to preserve the fidelity of signals in electronic hearing aid devices and electronic sound systems. According to the present invention, the noise suppression devices and methods utilize both analog and digital signal processing techniques.
2. The Prior Art
One of the most common complaints made by hearing aid users is the inability to hear in the presence of noise. Accordingly, the suppression of noise has long been the focus of researchers, and many approaches to solving the noise suppression problem have been proposed. In one approach, an independent measure of the noise is made and then subtracted from the signal being processed. This technique is typically applied to signals that are expressed as follows:s(t)=d(t)+n(t)
Wherein s(t) is the signal being processed, d(t) is the desired portion of the signal s(t), and n(t) the noise in the signal s(t).
For example, one or more sensors may be employed along with adaptive techniques to form an independent measure of the estimate of the noise, ne(t) from interference. By subtracting the noise estimate, ne(t), from the signal, s(t), an improved version of the desired signal, d(t), is obtained. To emphasize the subtraction of the noise estimate, ne(t), this technique is commonly referred to as “noise canceling.” This noise canceling technique has been applied to both sonar systems and medical fetal electrocardiograms, and has further been found to be effective to process acoustic signals containing both speech and interference. See for example, Douglas M. Chabries, et al., “Application of Adaptive Digital Signal Processing to Speech Enhancement for the Hearing Impaired,” Journal of Rehabilitation Research and Development, Vol. 24, No. 4, pp. 65-74, and Robert H. Brey, et al., “Improvement in Speech Intelligibility in Noise Employing an Adaptive Filter with Normal and Hearing-Impaired Subjects,” Journal of Rehabilitation Research and Development, Vol., 24, No. 4, pp. 75-86.
When no independent sample or estimate of the noise is available, other techniques to provide noise suppression have been employed. In several instances, researchers have exploited the differences in the temporal properties of speech and noise to enhance the intelligibility of sound. These techniques are typically referred to as noise suppression or speech enhancement. See for example, U.S. Pat. No. 4,025,721 to Graupe, U.S. Pat. No. 4,185,168 to Graupe, and S. Boll, “Suppression of Acoustic Noise in Speech Using Spectral Subtraction,” IEEE Trans. on ASSP, Vol. ASSP-27, pp. 113-120, April 1979, H. Sheikhzadeh, et al., “Comparative Performance of Spectral Subtraction and HMM-Based Speech Enhancement Strategies with Application to Hearing Aid Design,” Proc. IEEE ICASSP, pp. I-13 to I-17, 1994, and P. M Crozier, B M G Cheethan, C. Holt, and E. Munday, “Speech enhancement employing spectral subtraction and linear predictive analysis,” Electronic Letters, 29(12):1094-1095, 1993.
These approaches have been shown to enhance particular signals in comparison to other signals that have been defined as noise. One researcher, Mead Killion, has noted that none of these approaches has enhanced speech intelligibility. See Mead Killion, Etymotic Update, Number 15, Spring 1997. However, in low noise environments, compression techniques have been shown to relieve hearing deficits. See Mead Killion, “The SIN report: Circuits haven't solved the hearing-in-noise problem,” The Hearing Journal, Vol. 50, No. 20, October 1997, pp. 28-34.
With these techniques, researchers have generally noted a decrease in speech intelligibility testing when noise contaminated speech is processed, despite the fact that measures of quality or preference increase. Typically, the specification of the noise characteristics or the definition of the speech parameters distinguishes the various techniques in the second category of noise suppression from one another. It has been demonstrated that acoustic signals can be successfully processed according to these techniques to enhance voiced or vowel sounds in the presence of white or impulsive noise, however, these techniques are less successful in preserving unvoiced sounds such as fricatives or plosives.
Other noise suppression techniques have been developed wherein speech is detected and various proposed methods are employed to either turn off the amplifier in a hearing aid when speech is not present or to clip speech and then turn off the output amplifier in the absence of detectable speech. See for example, Harry Teder, “Hearing Instruments in Noise and the Syllabic Speech-to-Noise Ratio,” Hearing Instruments, Vol. 42, No. 2, 1991. Further examples of the approach to noise suppression by suppressing noise to enhance the intelligibility of sound are found in U.S. Pat. No. 4,025,721 to Graupe, U.S. Pat. No. 4,405,831 to Michaelson, U.S. Pat. No. 4,185,168 to Graupe et al., U.S. Pat. No. 4,188,667 to Graupe et al., U.S. Pat. No. 4,025,721 to Graupe et al., U.S. Pat. No. 4,135,590 to Gaulder, and U.S. Pat. No. 4,759,071 to Heide et al.
Other approaches have focussed upon feedback suppression and equalization (U.S. Pat. No. 4,602,337 to Cox, and U.S. Pat. No. 5,016,280 to Engebretson, and see also Leland C. Best, “Digital Suppression of Acoustic Feedback in Hearing Aids, ” Thesis, University of Wyoming, May 1995 and Rupert L. Goodings, Gideon A. Senensieb, Phillip H. Wilson, Roy S. Hansen, “Hearing Aid Having Compensation for Acoustic Feedback,” U.S. Pat. No. 5,259,033 issued Nov. 2, 1993.), dual microphone configurations (U.S. Pat. No. 4,622,440 to Slavin and U.S. Pat. No. 3,927,279 to Nakamura et al.), or upon coupling to the ear in unusual ways (e.g., RF links, electrical stimulation, etc.) to improve intelligibility. Examples of these approaches are found in U.S. Pat. No. 4,545,082 to Engebretson, U.S. Pat. No. 4,052,572 to Shafer, U.S. Pat. No. 4,852,177 to Ambrose, and U.S. Pat. No. 4,731,850 to Levitt.
Still other approaches have opted for digital programming control implementations which will accommodate a multitude of compression and filtering schemes. Examples of such approaches are found in U.S. Pat. No. 4,471,171 to Kopke et al. and U.S. Pat. No. 5,027,410 to Williamson. Some approaches, such as that disclosed in U.S. Pat. No. 5,083,312 to Newton, utilize hearing aid structures which allow flexibility by accepting control signals received remotely by the aid.
U.S. Pat. No. 4,187,413 to Moser discloses an approach for a digital hearing aid which uses an analog-to-digital converter and a digital-to-analog converter, and implements a fixed transfer function H(z). However, a review of neuro-psychological models in the literature and numerous measurements resulting in Steven's and Fechner's laws (see S. S. Stevens, Psychophysics, Wiley 1975; G. T. Fechner, Elemente der Psychophysik, Breitkopf u. Härtel, Leipzig, 1960) conclusively reveals that the response of the ear to input sound is nonlinear. Hence, no fixed linear transfer function H(z) exists which will fully compensate for hearing.
U.S. Pat. No. 4,425,481 to Mansgold, et. al. discloses a programmable digital signal processing (DSP) device with features similar or identical to those commercially available, but with added digital control in the implementation of a three-band (lowpass, bandpass, and highpass) hearing aid. The outputs of the three frequency bands are each subjected to a digitally controlled variable attenuator, a limiter, and a final stage of digitally controlled attenuation before being summed to provide an output. Control of attenuation is apparently accomplished by switching in response to different acoustic environments.
U.S. Pat. Nos. 4,366,349 and 4,419,544 to Adelman describe and trace the processing of the human auditory system, but do not reflect an understanding of the role of the outer hair cells within the ear as a muscle to amplify the incoming sound and provide increased basilar membrane displacement. These references assume that hearing deterioration makes it desirable to shift the frequencies and amplitude of the input stimulus, thereby transferring the location of the auditory response from a degraded portion of the ear to another area within the ear (on the basilar membrane) which has adequate response.
Mead C. Killion, The k-amp hearing aid: an attempt to present high fidelity for persons with impaired hearing, American Journal of Audiology, 2(2): pp. 52-74, July 1993, states that based upon the results of subjective listening tests for acoustic data processed with both linear gain and compression, either approach performs equally well. It is argued that the important factor in restoring hearing for individuals with hearing losses is to provide the appropriate gain. In the absence of a mathematically modeled analysis of that gain, several compression techniques have been proposed, e.g., U.S. Pat. No. 4,887,299 to Cummins; U.S. Pat. No. 3,920,931 to Yanick, Jr.; U.S. Pat. No. 4,118,604 to Yanick, Jr.; U.S. Pat. No. 4,052,571 to Gregory; U.S. Pat. No. 4,099,035 to Yanick, Jr. and U.S. Pat. No. 5,278,912 to Waldhauer. Some involve a linear fixed high gain at soft input sound levels and switch to a lower gain at moderate or loud sound levels. Others propose a linear gain at soft sound intensities, a changing gain or compression at moderate intensities and a reduced, fixed linear gain at high or loud intensities. Still others propose table look-up systems with no details specified concerning formation of look-up tables, and others allow programmable gain without specification as to the operating parameters.
Switching between the gain mechanisms in each of these sound intensity regions has introduced significant distracting artifacts and distortion in the sound. Further, these gain-switched schemes have been applied typically in hearing aids to sound that is processed in two or three frequency bands, or in a single frequency band with pre-emphasis filtering.
Insight into the difficulty with prior art gain-switched schemes may be obtained by examining the human auditory system. For each frequency band where hearing has deviated from the normal threshold, a different sound compression is required to provide normal hearing sensation. Therefore, the application of gain schemes which attempt to use a frequency band wider than a single critical band (i.e., critical band as defined in “Fundamentals of Hearing, An Introduction,” Third Edition by William A. Yost, Academic Press, 1994, page 307) cannot produce the optimum hearing sensation in the listener. If, for example, it is desired to use a frequency bandwidth which is wider than the bandwidth of the corresponding critical bandwidth, then some conditions must be met in order for the wider bandwidth to optimally compensate for the hearing loss. These conditions are that the wider bandwidth must exhibit the same normal hearing threshold and dynamic range and require the same corrective hearing gain as the critical bands contained within the wider bandwidth. In general, this does not occur even if a hearing loss is constant in amplitude across several critical bands of hearing. Failure to properly account for the adaptive full-range compression will result in degraded hearing or equivalently, loss of fidelity and intelligibility perceived by the hearing impaired listener. Therefore, mechanisms as disclosed, which do not provide a sufficient number of frequency bands to compensate for hearing losses, will produce sound which is of less benefit to the listener in terms of the quality (user preference) and intelligibility.
Several schemes have been proposed which use multiple bandpass filters followed by compression devices (see U.S. Pat. No. 4,396,806 to Anderson, U.S. Pat. No. 3,784,750 to Stearns et al., and U.S. Pat. No. 3,989,904 to Rohrer).
One example of prior art in U.S. Pat. No. 5,029,217 to Chabries focused on a Fast Fourier Transform (FFT) frequency domain version of a human auditory model. As known to those skilled in the art, the FFT can be used to implement an efficiently-calculated frequency domain filter bank which provides fixed filter bands. As described herein, it is preferred to use bands that approximate the critical band equivalents which naturally occur in the ear due to its unique geometry and makeup. The use of critical bands for the filter bank design allows the construction of a hearing aid which employs wider bandwidths at higher frequencies while still providing the full hearing benefit. Because the resolution of the FFT filter bank must be set to the value of the smallest bandwidth from among the critical bands to be compensated, the efficiency of the FFT is in large part diminished by the fact that many additional filter bands are required in the FFT approach to cover the same frequency spectrum. This FFT implementation is complex and likely not suitable for low-power battery applications.
As known to those skilled in the art, prior-art FFT implementations introduce a block delay by gathering and grouping blocks of samples for insertion into the FFT algorithm. This block delay introduces a time delay into the sound stream which may be long enough to be annoying and to induce stuttering when one tries to speak. An even longer delay could occur which sounds like an echo when low levels of compensation are required for the hearing impaired individual.
For acoustic input levels below hearing threshold (i.e. soft background sounds which are ever present), the FFT implementation described above provides excessive gain. This results in artifacts which add noise to the output signal. At hearing compensation levels greater than 60 dB, the processed background noise level can become comparable to the desired signal level in intensity, thereby introducing distortion and reducing sound intelligibility.
As noted above, the hearing aid literature has proposed numerous solutions to the problem of hearing compensation for the hearing impaired. While the component parts that are required to assemble a high fidelity, full-range, adaptive compression system have been known since 1968, no one has to date proposed the application of the multiplicative AGC to the several bands of hearing to compensate for hearing losses.
As will be appreciated by those of ordinary skill in the art, there are three aspects to the realization of a high effectiveness aid for the hearing impaired. The first is the conversion of sound energy into electrical signals. The second is the processing of the electrical signals so as to compensate for the impairment of the particular individual which includes the suppression of noise from the acoustic signal being input to a hearing aid user while preserving the intelligibility of the acoustic signal. Finally, the processed electrical signals must be converted into sound energy in the ear canal.
Modern electret technology has allowed the construction of extremely small microphones with extremely high fidelity, thus providing a ready solution to the first aspect of the problem. The conversion of sound energy into electrical signals can be implemented with commercially available products. A unique solution to the problem of processing of the electrical signals to compensate for the impairment of the particular individual is set forth herein and in parent application Ser. No. 08/272,927 filed Jul. 8, 1994, now U.S. Pat. No. 5,500,902. The third aspect has, however, proved to be problematic, and is addressed by the present invention.
An in-the-ear hearing aid must operate on very low power and occupy only the pace available in the ear canal. Since the hearing-impaired individual has lower sensitivity to sound energy than a normal individual, the hearing aid must deliver sound energy to the ear canal having an amplitude large enough to be heard and understood. The combination of these requirements dictates that the output transducer of the hearing aid must have high efficiency.
To meet this requirement transducer manufacturers such as Knowles have designed special iron-armature transducers that convert electrical energy into sound energy with high efficiency. To date, this high efficiency has been achieved at the expense of extremely poor frequency response.
The frequency response of prior art transducers not only falls off well before the upper frequency limit of hearing, but also shows resonances starting at about 1 to 2 kHz, in a frequency range where they confound the information most useful in understanding human speech. These resonances significantly contribute to the feedback oscillation so commonly associated with hearing aids, and subject signals in the vicinity of the resonant frequencies to severe intermodulation distortion by mixing them with lower frequency signals. These resonances are a direct result of the mass of the iron armature, which is required to achieve good efficiency at low frequencies. In fact it is well known to those of ordinary skill in the art of transducer design that any transducer that is highly efficient at low frequencies will exhibit resonances in the mid-frequency range.
A counterpart to this problem occurs in high-fidelity loudspeaker design, and is solved in a universal manner by introducing two transducers, one that provides high efficiency transduction at low frequencies (a woofer), and one that provides high-quality transduction of the high frequencies (a tweeter). The audio signal is fed into a crossover network which directs the high frequency energy to the tweeter and the low frequency energy to the woofer. As will be appreciated by those of ordinary skill in the art, such a crossover network can be inserted either before or after power amplification.
From the above recitation, it should be appreciated that many approaches have been taken in the hearing compensation art to improve the intelligibility of the acoustic signal being input to the user of a hearing compensation device. These techniques include both compensating for the hearing deficits of the hearing impaired individual by various methods, and also for removing or suppressing those aspects of the acoustic signal, such as noise, that produce an undesirable effect on the intelligibility of the acoustic signal. Despite the multitude of approaches, as set forth above, that have been adopted to provide improved hearing compensation for hearing impaired individuals, there remains ample room for improvement.